Cationically curable compositions are well known in the art. Such compositions find utility as adhesives, coating agents, sealing agents and as moulding agents. Typically, cationically curable compositions are latent one-part, e.g. based on onium salt cationic initiators, which are activated thermally or photochemically, or two/multi-component systems that are activated at room temperature by mixing.
There are numerous reports in the patent literature of two-part compositions, a first part comprising a hardener and a second part comprising a cationically curable monomer, which undergo cure when both parts are mixed. For example, U.S. Pat. No. 4,396,754 discloses two-part epoxy compositions comprising Group II metal fluoroborate salts as efficient curing agents for epoxy-based curable monomers. U.S. Pat. No. 4,241,204, and International Patent Publication numbers WO8001695, and WO8002839 disclose two-part epoxy curable compositions where the curing agent is described as an onium salt/reducing agent/copper salt combination. Upon mixing both parts, the copper salt is reduced by the reducing agent which in turn reduces the onium salt, thereby initiating cure of the epoxy monomer. Separate parts of the compositions contain an integral component of the curing agent combination.
U.S. Pat. No. 4,717,440 of Drain et al. describes a two component cationically curable composition wherein a first part comprises a curable epoxy monomer and an organic halide and a second part comprises a curable epoxy monomer and a silver salt of a non-nucleophilic anion. Cure is achieved by mixing the first and second parts of the two-part composition.
Aside from the issue of packaging two separate compositions, two-part cationically curable compositions may suffer in that efficient mixing of both parts is often necessary prior to cure. Furthermore, it may be necessary for the user to vary the mixing ratios of the two components depending on the end use. Thus one-part cationically curable compositions may be desirable in situations where efficient mixing is not possible and/or to avoid mixing ratio problems.
Known methods of circumventing the problems associated with formulating both a curable monomer and a hardener into a one-part composition include activating a latent curing agent utilising heat or UV irradiation. For example, U.S. Pat. No. 3,759,914 describes heat sensitive latent amines incorporated into one-part epoxy compositions. U.S. Pat. No. 6,376,368 describes one-part epoxy compositions comprising UV sensitive latent curing agents. Irradiation of a composition comprising an N-benzylpyrazinium hexafluoroantimonate and an epoxy resin with UV light results in cure of the epoxy resin.
Primers are preparatory coatings placed on a surface to enhance or promote cure of a curable composition applied thereto. Primers are often utilised to improve adhesion to problematic substrates. Application of a primer to such problematic substrates can result in greatly improved adhesion to these substrates. Primers may be applied soon before the application of an adhesive, or parts may be pre-treated with a primer for storage with a view to subsequent application of an adhesive composition some time in the future. In the latter case the applied primer must stable such that it retains its efficacy over time.
Notwithstanding the state of the art it would be desirable to provide one-part cationically curable compositions with long term storage stability, wherein cure is independent of external stimuli such as heat and UV irradiation. In particular, it would be desirable to provide compositions with a long shelf life that will cure only upon application of the composition to the “target surface”, i.e. the surface is involved in initiating cure of the cationically curable component. It would additionally be desirable to render any surface as a “target surface” through provision of suitable primer compositions for priming surfaces to which cationically curable compositions can be applied.